Sleeping with the Fishes: the 400 million year old origin of sex

Generations of scientists have assumed that sexual reproduction in vertebrates originated with external fertilization that gradually evolved into the internal form that humans perform, and not the other way around. New findings published in Nature by Flinders University Professor John Long suggest otherwise. Long’s team has proposed that internal fertilization began around 400 million years ago in Placoderms, a class of prehistoric armored fish.

Newly discovered Placoderm fossils of the species Microbrachius dicki have shed light on the function of small “arms” on the sides of these fish. Microbrachius translates to small arms, but Professor Long’s study instead dubs these male projections as “claspers” that extend the same length as the protected cloaca of the females. Furthermore, grooves on the male portion are hypothesized to have been channels for delivering sperm. This differs markedly from the female fossils, which instead feature fixed plates to hold the claspers in place. Such a distinction marks the first instance of gender differentiated reproductive structures, which, oddly, is a trait these ancient fish share with humans.

In fact, other data collected by Long’s group suggest that Placoderms are not only the earliest example of sexual reproduction, but also the first vertebrate ancestor of Homo sapiens sapiens. Previously, these fish were thought to be an evolutionary dead end, but closer analysis of their jaws, teeth, and paired limbs, suggests that these features actually originated in Placoderms.

Though this study has challenged and reinvigorated the vertebrate archaeological field, still further research must be done to illuminate the origins of our evolutionary trail.